GT Cobra its hard to get info on our cars from these forums as most have the e38, pm and i'll give ya some links of forums and links to sites with good info on our e65/66 's. Ive been here for seen 2007 with my e66 and I assume I was the only one with one until more recently here. Now i'm seeing some people posting about these cars in here but again they are cheap to own now, but not cheap to repair.

Here's a quick copy/paste for ya:

E65/E66 (7 Series) equipped with HKL system (SA 316)

Troubleshooting Guide HKL system malfunctions.

A) The trunk lid will not open fully. The hydraulic pump makes a whining noise and the system is low on fluid.

CAUSE :
The material used in the hydraulic lines would allow the hydraulic fluid to evaporate through the lines when the trunk area was heated to very high temperatures, such as when the vehicle was parked in the sun. The fluid loss caused by this heating is gradual, and the system continues to work properly until the fluid gets low enough that the pump runs out of fluid before the lid is fully open or the fluid gets sufficiently aerated to cause the system to malfunction. Also, normal contraction of the hydraulic fluid due to low ambient temperatures could cause the system to malfunction in cooler weather because the fluid level in the reservoir drops below the critical level.

CORRECTION :

Remove the right side trunk trim and check the fluid level in the HKL reservoir to determine if it is low.Check the color of the printing on the hydraulic lines. If the printing is white, the hydraulic cylinder/lines assembly should be replaced with the improved parts. If the printing is yellow, the improved parts have already been installed and the system should be checked for leaks or other problems.

51 24 7 134 592 Cylinder/Line Kit
51 24 7 133 667 HKL hydraulic fluid

B) The trunk lid opens fully. It then drops slowly back to approximately half open.
----This situation only affects vehicles produced before June 2003. For vehicles produced from March 2004, the trunk lid may drop slightly when it is opened for the first time after programming, resetting, replacement of the control module, etc. This is a normal situation. After this initial setting, the trunk lid will remain fully open.

C) The trunk lid does not open or close fully.
This situation may happen intermittently depending on outside temperature. Noises may also be heard from the hydraulic pump.

CAUSE :
The connection between the hydraulic cylinder and the line is not tight.
---For vehicles produced from March 2003 until December 2003, the trunk lid opening or closing forces may be too low or the control module may have a coding error.

CORRECTION :
Check the hydraulic lines and the hydraulic fluid level. Also check the HKL control module coding index and replace or recode as necessary. Only control module with a coding index of 03 can be coded. Control module with a coding index of 02 cannot be coded and must be replaced.Do not replace the gas struts for this problem.This (HKL) module can be found on the right rear trunk area labeled HKL. You can see the coding index stamped on the control unit.

D) The trunk lid only moves in one direction.
When pressing the button to close the trunk lid, it tries to continue upwards after it has already been opened. In most cases,there will be a fault stored in HKL module.

CAUSE :
The module is defective.

CORRECTION :
61 35 6 980 008 HKL control module

E) The trunk lid does not stay open on a gradient of more than 10 degrees.
---affects vehicle produced up to October 2004.

CAUSE :
The trunk lid gas struts are not designed for parking on extreme gradients.